Various yarn supply devices supply yarn from a yarn supply storage winding on a drum, cage, or the like, in short, a yarn supply means in which the yarn is supplied under essentially slip-less conditions. The yarn supply drum is driven by a motor. The speed of the motor is easily controlled if the motor is a stepping motor or other motor in which the speed depends on the frequency, or repetition rate of pulses which are supplied to the motor. Suitable yarn guide elements are associated with the yarn supply means, including yarn tension sensors, such as spring-loaded bails or the like, in engagement with the yarn, and providing an output signal which controls the frequency of the power supply to the electric motor. A control system is provided, receiving the sensing signal from the yarn tension sensor, receiving a command signal, and, in turn, controlling the frequency, or pulse repetition rate of the power supply to the motor to maintain the yarn tension essentially constant and in conformity with the command value.
In a yarn supply apparatus of this kind known from U.S. Pat. No. 3,858,416, which is hereby incorporated by reference, the sensing means having a linearly movable yarn guide element that senses the yarn passing over it emit an analog tension signal that is representative of the yarn tension and is compared with a fixed command value tension.
The differential tension corresponding to the control deviation is amplified and processed for generating a positioning signal, which is supplied as a frequency signal to the frequency-controlled drive motor of the yarn supply element. A signal generator is associated with the drive motor, emitting a signal that is representative of the speed of the drive motor and hence of the quantity of yarn supplied per unit of time, and this signal can be compared--as a function of the position of a switchover element--with a synchronization signal converted into an analog tension, the synchronization signal being furnished by a second signal generator that measures the speed of the needle cylinder of the circular knitting machine equipped with the yarn supply apparatus. The control deviation thus obtained, corresponding to the differential tension, is processed by the controller into a corresponding control signal for the motor, so that the speed of the motor and hence the yarn supply speed are synchronized with the speed of the needle cylinder of circular knitting machine. By appropriate manual actuation of the switchover element, a constant yarn tension or a yarn supply quantity that is in a fixed ratio to the speed of the circular knitting machine can be controlled selectively.
The basic problem in this kind of yarn supply apparatus is that it is incapable of following along with rapid changes in yarn tension or yarn travel speed. For example, in yarn-striping operation in circular knitting machines, extremely fast changes in yarn utilization occur, which is also true to a lesser extent for Jacquard machines. For instance, in a conventional circular knitting machine yarn utilization during the striping operation is typically approximately 4 m/sec at a knitting feed. Now if the striping apparatus, in accordance with its program, removes the yarn travelling at this speed and at the same time inserts a new, as yet unprocessed and therefore non-moving yarn, then this yarn change takes place within fractions of milliseconds. This means that within this short period of time, the old yarn is brought from its yarn travel speed of approximately 4 m/sec to a stop, and the new yarn must be accelerated up to the full yarn supply speed of 4 m/sec within this same short period of time. The unavoidable inertia of the motor and yarn supply element alone precludes such rapid movement, resulting in impermissible peak yarn tensions. However, the discontinuance characteristic of the motor prevents its being shut down within the required short period of time, so that loops form in the yarn that is being brought to a stop in the yarn supply element, and the yarn tension breaks down completely.
A further factor is that in a frequency-controlled motor, that is, a synchronous motor or stepping motor, which is particularly advantageous because it can be controlled exactly, the frequency upon startup and shutdown cannot be increased arbitrarily quickly from the starting frequency to the frequency appropriate for steady state operation, nor decreased from this frequency to the stopping frequency, because otherwise the motor is no longer able, with its rotary speed, to follow up this rapid change in frequency; it falls out of step and stops.
For these reasons, the known yarn supply apparatus is suitable only for yarn-utilizing textile machines in which particularly fast or abrupt changes in the yarn consumption do not occur. This applies, for example, to circular knitting machines that process plain, non-patterned tubular goods.